1. Field of the Invention
This invention relates to a developer for developing electrostatic latent image used in electrophotographic copying machine, a printer and a facsimile machine, and, in particular, to a binary developer composed of a carrier consisting of magnetic particles having a small diameter and a magnetic toner.
2. Description of the Prior Art
In conventional printers and facsimile machines etc. using electro-photography, an electrostatic latent image corresponding to information is formed, for example, on a cylindrical photosensitive drum, a magnetic developer is attracted on and conveyed by a developing roller consisting of a non-magnetic sleeve and a permanent magnet member inside the sleeve facing the photosensitive drum to form a magnetic brush in the developing region, and the surface of the photosensitive drum on which the electrostatic latent image is formed is rubbed with the magnetic brush to produce a visible image as a toner image. In the most general method, the image is thermally fixed after the visible toner image is transferred on to a recording member (for example plain paper).
In this case, since a small amount of toner remains on the photosensitive drum, the image medium, even after the toner image is transferred onto the recording paper, a cleaning device is usually provided for removing the remaining toner. Therefore, on the one hand, a problem arises in that a space for the above cleaning device is required in the vicinity of the photosensitive drum, interfering with attempts to reduce the size of the imaging equipment as a whole.
On the other hand, an example of the method for downsizing the imaging equipment as a whole is the omission of the above cleaning device and the provision of a so-called developer cleaning device, which recovers toner remaining on the photosensitive drum and develops the electrostatic latent image at the same time in a region where the photosensitive drum and the developing roller face each other (see, e.g., PUPA 4-86878). In imaging equipment without this cleaning device, a magnetic developer consisting of the mixture of a toner and a spherical magnetic carrier is used.
When a spherical magnetic carrier is used as described above, however, since the specific surface area of the magnetic carrier is small, the area contacting the toner is inevitably small, causing problems in that the tribo-electric charge of the toner is small, the image density is low, and sharp images cannot be produced.
If the particle diameter of the magnetic carrier is reduced for forming finer images, even though a thin developer layer is formed and high-resolution, high-quality images are obtained, there still remain such problems in that the ability of the developing means to magnetically hold the magnetic carrier causes the increased scattering of the carrier, the vicinity of the developing means is contaminated, and the image quality deteriorates due to the adhesion of the carrier on to the photosensitive drum.
Since the magnetic toner composing the binary developer as described above contains magnetic powder, the magnetic toner on the image produced on the photosensitive drum moves easily due to the revolution of the developing roller, causing the problem of tailing which lowers the image quality.
Furthermore, when a developer cleaning device, which recovers the toner and develops the electrostatic latent image at the same time, is used for developing an image with each revolution of the drum, if the toner, after the toner image is transferred onto the recording paper, remains on the photosensitive drum, the remaining toner cannot be recovered completely by the developer cleaning section, and may adhere to and remain on the electrostatic latent image formation section even after developing.
If the remaining toner has not been recovered completely, the quality of the image obtained significantly deteriorates. In order to solve such problems, a method for developing an image with each two revolutions of the photosensitive drum may be used to recover the remaining toner as described above completely but, in such a system, the speed of image production is inevitably lowered, and the requirement for quick information transmission may not be met.